Municipal and industrial water treatment systems play a crucial role in an environmentally sustainable society. Water treatment in such systems is conventionally divided into three stages: primary, secondary and advanced water treatment. Advanced water treatment is defined herein as including tertiary water treatment, physical-chemical treatment, combined biological-physical treatment, and generally any process designed to produce an effluent of higher quality than normally achieved by secondary treatment processes. Advanced water treatment improves the quality of water prior to reuse or discharge into the environment in order to limit adverse environmental impact and/or comply with governmental regulations or other restrictions. Advanced treatment typically includes, for example, reducing excessive levels of nutrients (such as nitrogen and phosphorus), as well as reducing trace metals (such as selenium and magnesium) and other pollutants and regulated compounds (such as polychlorinated biphenyls, biological oxygen demand, and carbonaceous oxygen demand). The amount of water discharge, and in a growing number of cases re-use, is limited by the quality of the water. Consequently, there is a need for water treatment systems that are capable of producing water that meets regulated discharge or re-use requirements.
One approach to water treatment involves the use of algae or other biological agents such as cyanobacteria to remove or reduce the amounts of nutrients and trace metals. Existing systems of this kind, however, have one or more disadvantages that include:                a) Low flow rates which require a significant physical footprint to treat large volumes of water,        b) Unacceptably high levels of nutrients, metals, or other pollutants in the treated water,        c) Limited light availability/penetration to optimize biological processes,        d) Lack of velocity resulting in bio-film build up,        e) Uncontrolled environments (open ponds/raceways) presenting opportunities for easy contamination or change in water chemistry,        f) Suspended solids buildup due to lack of pre-filtration, and        g) Inability to manage the bioavailability of constituents in the water for removal by the biological agent (algae, cyanobacteria, etc.).        
Accordingly, there remains a need for improved techniques for advanced water treatment using algae or other biological organisms.